Perianal Paget's disease is an uncommon intraepidermal carcinoma characterized by the presence ofPaget cells. It usually affects older patients and commonly presents as chronic perianal pruritus with scaly plaques. The disease is categorized into primary perianal Paget's disease ofcutaneous origin and secondaryperianal Paget's disease, which is due to extension of a visceral malignancy such as that of the anorectum or colon. Cytokeratin 7 (CK7), cytokeratin 20 (CK20), and gross cystic disease fluid protein-15 (GCDFP15) expression are useful for differentiation between these two types. A tumor immunohistochemical profile of CK7+/CK20-/GCDFP15+ suggests the primary type, whereas CK7+/CK20+/GCDFP15- suggests the secondary type. The expression of caudal homeobox 2 (CDX2) suggests the secondary type from anorectal or colonic adenocarcinoma. However, approximately one- third of patients without visceral malignancy have a tumor that is CK7+/CK20+/GCDFP15-. Two percents of primary perianal Paget ' disease can express CDX2. The author reports a case ofan 86-year-old man who presented with chronic perianalpruritus and a scaly plaque. A skin biopsy showed intraepidermal Paget cells with immunohistochemical profile of CK7+/CK20+/GCDFPJ5-/CDX2+. Initially, secondary perianal Paget's disease from colorectal adenocarcinoma was suspected. However, extensive investiga- tions found no visceral malignancy. The patient underwent wide excision of the perianal skin. Pathological examination showed diffuse intraepidermal Paget cells withfocal dermal invasion by intestinal-type adenocarcinoma and signet-ring cell differentiation. In conclusion, thefinal diagnosis was primary perianal Paget's disease withfocal adenocarcinona and signet- ring cell differentiation. The disease was consistent with primary perianal Paget's disease, because no visceral malignancy was found.